Our reliance on the use of crude oil as a source of relatively cheap energy is ever increasing. Developed and developing countries continue to consume record volumes of this limited resource to sustain their economy. At the same time the impact to the environment, in the form of air pollution and greenhouse gases, is alarming.
Oil deposits are being depleted at a high rate and demand is still expected to grow as nations' requirement for energy continues to increase. According to industry experts, the use of motor vehicles as a mode of transportation contributes to more than 10% of all greenhouse gases emitted to the environment. The number may not imply a significant figure when compared to other contributing factors but combined with the fact that the oil required to power these vehicles is dwindling in quantity, and the growing risk of accelerating the greenhouse effect, it is important to identify methods to run vehicles more efficiently.
Although alternative methods to power motor vehicle are becoming available, a large percentage of cars on the roads today are still powered by internal combustion engines. The use of an internal combustion engine is possibly the least efficient way to power a vehicle. Most engines were designed during the time when fuel was abundant and the effects of pollution and emission were not a major concern.
An internal combustion engine is theoretically a heat pump. The efficiency of a heat pump is measured by its ability to produce work (output) relative to the heat used (input). The input is generally derived from the combustion of fossil fuels. Output is measured as the mechanical work produced to propel wheels of the vehicle minus the losses incurred in the process.
There are several factors that can contribute to the efficiency of an engine: combustion, heat transfer, friction, type of fuel, exhaust gas scavenging and ignition process. While not all of these factors can be addressed in a single solution, aspects of the implementation of an internal combustion engine could certainly be improved.
There are a large number of notable enhancements to the design of the internal combustion engine during the last 30 years. Development in technologies related to metallurgy, chemistry and electronics has allowed engineers and designers to reduce some issues with losses. Improvements in manufacturing processes have similarly contributed in producing finely tuned engines with higher operating efficiencies.
A typical engine design employs a combination of openings, ports and valves to facilitate the flow of air/fuel and other gases in and out of a combustion chamber. On typical two-stroke engines, openings on the cylinder wall are used as intake and exhaust passages. In contrast, typical four-stroke engines utilise ports in combination with valves to control the flow of fluids in and out of the combustion chamber.
There are a number of drawbacks with these traditional methods. For example, in a two-stroke engine, the exhaust gases from the combustion chamber are not expelled fully as they cannot flow freely out of the combustion chamber. This leaves combusted air/fuel in the combustion chamber further reducing the efficiency of any subsequent combustion therein. Even with modern two-stroke engines the combination of openings and port utilised produces losses that drive efficiency down.
Due to their simplicity two-stroke engines have less moving parts than four stoke engines; however, their simplicity limits the positioning of certain components. In contrast a four-stroke engine does not suffer from the same limitations in positioning components but the added componentry introduces further efficiency losses to the complexity of the engine. For example, actuating a valve requires a number of additional mechanical components. For every component introduced, extra losses are incurred. Furthermore, these valves can introduce geometric limitations to the port locations on the cylinder head. This results in the losses from inefficient exhaust gas scavenging, which is apparent with typical engine designs.
The most popular types of engines are the petrol (Homogenous Charge Spark Ignition—HCSI) and diesel (Stratified Charge Compression Ignition—SCCI) engine. While they are not highly efficient engines by design, both are widely used because they are well understood and their implementation is cost effective. The petrol and diesel engine both have their own limitations and are not the most frugal to run.
While advancement in automotive technology contributes incremental improvements, it would be desirable to further improve the running efficiency of engines. To this end a hybrid ignition system, referred to as Homogeneous Charge Compression Ignition (HCCI) engine has emerged, offering improvements over typical HCSI and SCCI engines.
HCCI is essentially a variation of an Otto engine and operates similarly to a petrol engine except for the method of ignition. HCCI is a combination of the two combustion strategies applied in gasoline and diesel engines, in which ignition is achieved by compressing the air/fuel mixture in the cylinder until it spontaneously ignites.
The implementation of HCCI on internal combustion engines is highly desirable as estimates from industry indicate efficiencies of around 45% are achievable and with further development this could reach the 60% industry target.
The thermochemical reaction during combustion in HCCI is preferred because it is achieved at a lower temperature. This is advantageous as it reduces the production of NOx and soot. HCCI also runs on a lean fuel and with a high compression ratio, which equates to lower fuel consumption and higher output power. HCCI engines, due to the ignition process are capable of running on petrol (gasoline), diesel and other fuel mixtures.
However, the HCCI ignition process is not without drawbacks. First, it tends to be problematic and difficult to control. The mixture of air and fuel can auto-ignite anytime should the right conditions occur in the cylinder. Secondly, the pressure in the HCCI cylinder is also high compared to a typical HCSI engine which can result in heavy engine wear and a risk of potential engine failure. Thirdly, the HCCI ignition process can result in increased carbon monoxide (CO) and hydrocarbon (HC) emissions due to incomplete oxidation during the combustion process.
Typical HCCI engines are prone to quality control issues such as detonation, which has a negative impact on the engine's performance and reliability. Diesel engines typically counter detonation by introducing the fuel to the fuel air mixture very late in the piston stroke, immediately prior to ignition thereby reducing allowable time in which detonation can occur.
Detonation can reduce the effective operating range of an engine while operating in HCCI mode. At the low end of the engine operating range, detonation results in poor response due to lack of available torque and at the high end of the operating range the probability of detonation increases.
The term “detonation” is understood herein to refer to is the spontaneous detonation combustion of air and fuel mixture outside the prescribed ignition point in the engine cycle. Detonation is also referred to pinging, pinking or engine knock.
The impact of this detonation ranges from minor to major depending on when it occurs and the extent of the event. The shockwave created during detonation exhibits a distinctive “ping” sound, which is produced by the standing waves bouncing within the cylinder. The impact of engine knock is minimal if the amount of air/fuel mixture constitutes only of a small percentage of the total volume of gases in the cylinder. If the air/fuel mixture constitutes a substantial proportion of the total volume of gases in the cylinder, the excessive pressure developed during detonation can destroy components of the engine and result in engine failure.
The effect of detonation also affects the overall performance of the engine. Detonation prevents the engine from delivering the required power at higher operating speeds. It can also diminish engine responsiveness at the low end of the operating range due to reduced torque. As the engine is loaded or worked harder, the tendency for detonation to occur is increased as a result of higher pressure in the cylinders.
As global consumption of fossil fuels increases, a viable method of operating an internal combustion engine is through the implementation of HCCI. Combustion in a HCCI engine occurs when the mixture of air and fuel in the combustion chamber is compressed until the components of the mixture spontaneously react and ignite. The increase in pressure in the combustion chamber (ie decrease in volume) increases the temperature of the air/fuel mixture forcing the mixture to combust. Due to the nature of this type of ignition, it is inherently difficult to predict and control the exact point in time when this event occurs.
The composition of the fuel/air mixture also affects the ignition process. Varying the octane rating of a fuel can vary the point of ignition in the process. Similarly, the amount of external gases introduced in the combustion chamber can influence the ignition event. The combination of all of these elements creates a challenge for controlling the exact point at which ignition will occur. Control of the ignition event is important as this is the primary mechanism used to manipulate the response and power delivery of an engine.
Attempts to control ignition in a HCCI engine exploit the different parameters that influence the ignition event: temperature, pressure, external gases and composition of the fuel. These parameters can be manipulated to either delay or advance the ignition point. However, the management of the above variables across the operating parameters of the engine requires a very complex algorithm. A computerised electronic control system can provides a solution but the cost involved is prohibitive for most commercial purposes.
There is a need to deliver an effective method of operating a HCCI engine, of controlling the ignition event, reducing emissions and reducing the probability of detonation.
In light of the above drawbacks it would be advantageous to provide a more efficient means of implementing an HCCI internal combustion engine.